1. Field of the Invention
The present invention relates to electromagnetic receivers and more particularly to global position receivers employing ground based antennas to receive signals from transmitters positioned on satellites.
2. Description of the Prior Art
In the prior art, ground based receivers for use in global position systems are known. In such systems, a ground based antenna is positioned to receive signals transmitted from satellites and to process such signals for transmission to remote receivers such as located on aircraft for use in determining position. A problem encountered with ground based antennas is that multipath propagation of electromagnetic signals causes errors in the signals produced by the ground based receivers. More particularly, reflections of radiation from the ground and from objects which are above the antenna's horizon cause unwanted interference with the signals received directly from the satellites. Several solutions to this problem have been proposed. For example, in copending application Ser. No. 08/294,519 of Robert Gille, filed on even date herewith and assigned to the assignee of the present invention, protective materials such as choke coils and absorbing materials are placed around the antenna to inhibit the ground and object reflections.
While this provides satisfactory protection in most cases, the output of the antenna may still be in error with the result that erroneous signals may be transmitted to the aircraft.